Multilayer film having a layer of low pressure, low density heterogeneous ethylene copolymers

ABSTRACT

A multilayer plastic film suitable for use in the manufacture of garbage and trash bags wherein the film comprises at least one layer of a high pressure low density polyethylene, and a layer comprising low pressure low density gas phase heterogeneous copolymers formed from ethylene, and at least one of propylene and butene-1 and at least one C 5  -C 8  comonomer.

This application is a division of prior U.S. application Ser. No.317,936, filed Nov. 3, 1981, now U.S. Pat. No. 4,464,426.

This invention relates to a multilayer plastic film, and moreparticularly, to a multilayer plastic film suitable for the manufactureand use as garbage and trash bags.

In general, plastic garbage and trash bags for bulk waste material maybe produced in film form from various polymers such as polyethylene. Thefilms used for such bags should desirably possess high strengthcharacteristics such as puncture toughness, tensile strength andElmendorf tear strength. Another desirable property of plastic bags islow manufacturing cost. It will be recognized that the required quantityof raw material may be reduced by use of polyethylene film having thepreviously mentioned high strength characteristics, and in part for thisreason, multiple layer plastic films have been used to manufacturegarbage bags.

With the introduction of "linear" low density polyethylenes made by thelow pressure processes (hereinafter referred to as "LPLDPE"), attemptshave been made to substitute this material for film applications inplace of conventional highly branched low density polyethylenes made byhigh pressure processes (hereinafter referred to as "HPLDPE"). Thereason for these efforts is that LPLDPE is widely recognized as beingtougher and stronger than HPLDPE. However, it has been discovered thatthe machine direction (MD) Elmendorf tear strength for LPLDPE film issubstantially lower than HPLDPE film, even though other physicalproperties of LPLDPE are superior.

The prior art polyethylene film-type bags have thus been characterizedby either limited strength properties except for high MD tear strength,with relatively low extruder power requirement and low extruder headpressure on one hand, or characterized by high strength properties, butwith only moderate MD tear strength, and relatively high extruder powerrequirement and high extruder head pressure on the other hand.

In addition, polyethylene film for the production of consumer garbageand trash bags is generally prepared in the thickness range of betweenabout 1.5 mils and about 3.0 mils. As earlier indicated, it is customaryfor low density polyethylene, produced by the standard high pressureprocess, to be used as the basic polymer or raw material for thisproduct because of its relatively low cost, good physical properties,ease of extrusion at high rates, good heat seal strength, and ability tobe readily converted into the finished product.

There is, however, a continuous need for stronger films for thisapplication which will be even more resistant to damage by puncture,tearing, or yielding under stress. A stronger film is not only desirablefrom the standpoint that the finished bag is more serviceable in theend-use, but also in that a thinner film can be used and still meet thenecessary strength requirements, thus providing a bettercost-performance relationship for the consumer.

Accordingly, it is an object of this invention to provide a multi-layerpolyethylene film possessing improved puncture toughness, tensilestrength, yield strength and tear strength over conventional multi-layerpolyethylene films.

It is another object of this invention to provide a multi-layerpolyethylene film having higher transverse direction tear strength thanhigh pressure, low density polyethylene.

It is another object of this invention to provide a multi-layerpolyethylene film having a relatively higher machine direction tearstrength than low density copolymers formed from monomers such asethylene and butene.

These objects, and others, will be apparent from a reading of thefollowing description of the invention and the appended claims.

In accordance with the present invention, there is provided amulti-layer film suitable for use in the manufacture of garbage andtrash bags which, in one embodiment, comprises a first outer layer of ahigh pressure low density polyethylene, a core layer of low pressure,low density gas phase heterogeneous ethylene copolymers; and a secondouter layer of a high pressure low density polyethylene. It has beenfound that the multilayer film of this invention has physicalproperties, such as puncture toughness, tensile strength, particularlytransverse machine direction tensile strength and yield strength, whichare substantially improved over those of conventional multilayer filmsmade from high pressure, low density polyethylene in all layers thereof.Surprisingly, the machine direction tear strength observed for theco-extruded multilayer film remains high even as the density of theheterogeneous ethylene copolymers is increased, from 0.917 to 0.928 asshown in the examples and subsequent discussion relative thereto. It isexpected, ordinarily, that as density is increased, machine directiontear strength decreases in a blown film of this type. A higher machinedirection tear strength results in a more balanced film (machinedirection versus transverse direction) which is highly desirable from anend-use standpoint. In addition, the fact that the machine directiontear strength is not lowered as density is increased allows the use ofthe higher density copolymers with their higher modulus and higher yieldstrength.

Consequently, it has been found that a multilayered, coextruded filmcomprising the low pressure, low density gas phase heterogeneouscopolymer resins of this invention as one or more layers, and highpressure, low density polyethylene or blends of high pressure, lowdensity polyethylene and low pressure, low density gas phaseheterogeneous copolymer resins as one or more outer layers of theco-extruded film results in a superior, stronger film and trash bag.

The multilayered structure having high pressure, low densitypolyethylene as an outer layer is particularly desirable in order togain freedom from melt fracture of the layer of copolymers at highoutput rates and to aid in increasing the melt strength of the extrudatein order to improve bubble or melt cooling at higher output rates.

The copolymer layer of the multilayer film of this invention comprisesheterogeneous copolymers formed from ethylene, and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer. By the termheterogeneous, it is meant that the comonomer units are distributedrandomly along a polymer chain but do not have the sameethylene/comonomer ratio among the polymer molecules. The ethylenecopolymers are interpolymers or copolymers of the monomers C₂, C_(a) andC_(b), wherein C₂ is ethylene, C_(a) is selected from propylene,butene-1 and mixtures thereof, and C_(b) is selected from one or more ofthe C₅ to C₈ alpha monoolefins which contain no branching closer thanthe fourth carbon atom. The C₅ to C₈ monomers include pentene-1,4-methyl pentene-1, hexene-1, heptene-1 and octene-1.

The C₂, C_(a) and C_(b) monomer units are believed to be randomlydistributed along the polymer chain and do not have the sameethylene/comonomer ratio among the polymer molecules. The molar ratio ofC_(a) /C₂ monomer units in the polymer mass is about 0.006 to 0.09. Themolar ratio of C_(b) /C₂ monomer units in the polymer mass is about0.003 to 0.07. In addition, the C_(a) and C_(b) monomers are also usedin such amounts as to provide a Branch Chain Factor value of about 0.1to 0.9 and preferably of about 0.2 to 0.8, where the ##EQU1##

The copolymers have a density of about 0.91 to 0.94,

a melt flow ratio of ≧22 to 23 ∓and preferably of about ≧25 to ≦32, and

a melt index of about 0.5 to 5.0.

The melt flow ratio value is one means of indicating the molecularweight distribution of a polymer. The melt flow ratio (MFR) range of ≧22to ≦36 thus corresponds to M_(w) /M_(n) value range of about 2.7 to 4.3and the MFR range of ≧25 to ≦32 corresponds to a M_(w) /M_(n) range ofabout 2.8 to 3.8.

The copolymers of the present invention have an unsaturated groupcontent of ≦1, and usually of ≧0.1 to ≦0.6, C=C/1000 carbon atoms.

The copolymers can be readily produced in a low pressure gas phase fluidbed reaction process, as disclosed below, if a specific monomer chargeis polymerized under a specific set of operating conditions, asdisclosed below, and in the presence of a specific high activitycatalyst, which is also described below.

HIGH ACTIVITY CATALYST

The compounds used to form the high activity catalyst used in thepresent invention comprise at least one titanium compound, at least onemagnesium compound, at least one electron donor compound, at least oneactivator compound, and at least one porous inert carrier material, asdefined below.

The titanium compound has the structure

    Ti(OR).sub.a X.sub.b

wherein

R is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical, or COR'where R' is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical,

X is selected from the group consisting of Cl, Br, I, or mixturesthereof, a is 0 or 1, b is 2 to 4 inclusive and a+b=3 or 4.

The titanium compounds can be used individually or in combinationsthereof, and would include TiCl₃ and TiCl₄, Ti(OCH₃)Cl₃, Ti(OC₆ H₅)Cl₃and Ti(OCOC₆ H₅)Cl₃.

The magnesium compound has the structure

    MgX.sub.2

wherein X is selected from the group consisting of Cl, Br, I, ormixtures thereof. Such magnesium compounds can be used individually orin combinations thereof and would include MgCl₂, MgBr₂ and MgI₂.Anhydrous MgCl₂ is the particularly preferred magnesium compound.

About 0.5 to 56, and preferably about 1 to 10, mols of the magnesiumcompound are used per mol of the titanium compound in preparing thecatalysts employed in the present invention.

The titanium compound and the magnesium compound should be used in aform which will facilitate their dissolution in the electron donorcompound, as described herein below.

The electron donor compound is an organic compound which is liquid at25° C. and in which the titanium compound and the magnesium compound arepartially or completely soluble. The electron donor compounds are knownas such, or as Lewis bases.

The electron donor compounds would include such compounds as alkylesters of aliphatic and aromatic carboxylic acids, aliphatic ethers,cyclic ethers and aliphatic ketones. Among these electron donorcompounds the preferable ones are alkyl esters of C₁ to C₄ saturatedaliphatic carboxylic acids, alkyl esters of C₇ to C₈ aromatic carboxylicacids; C₂ to C₈, and preferably C₃ to C₄, aliphatic ethers; C₃ to C₄cyclic ethers, and preferably C₄ cyclic mono- or di-ether; C₃ to C₆,preferably C₃ to C₄, aliphatic ketones. The most preferred of theseelectron donor compounds would include methyl formate, ethyl acetate,butyl acetate, ethyl ether, hexyl ether, tetrahydrofuran, dioxane,acetone and methyl isobutyl ketone.

The electron donor compounds can be used individually or in combinationsthereof.

About 2 to 85, and preferably about 3 to 10 mols of the electron donorcompound are used per mol of Ti.

The activator compound has the structure

    Al(R").sub.c X'.sub.d H.sub.e

wherein X' is Cl or OR'", R" and R'" are the same or different and areC₁ to C₁₄ saturated hydrocarbon radicals, d is 0 to 1.5, e is 1 or 0 andc+d+e=3.

Such activator compounds can be used individually or in combinationsthereof and would include Al(C₂ H₅)₃, Al(C₂ H₅ Cl)Al(i--C₄ H₉)₃, Al₂ (C₂H₅)₃ Cl₃, Al(i--C₄ H₉)₂ H, Al(C₆ H₁₃)₃, Al(C₈ H₁₇)₃, Al(C₂ H₅)₂ H andAl(C₂ H₅)₂ (OC₂ H₅).

About 10 to 400, and preferably about 10 to 100, mols of the activatorcompound are used per mol of the titanium compound in activating thecatalyst employed in the present invention.

The carrier materials are solid, particulate porous materials which areinert to the other components of the catalyst composition, and to theother active components of the reaction system. These carrier materialswould include inorganic materials such as oxides of silicon and/oraluminum. The carrier materials are used in the form of dry powdershaving an average particle size of about 10 to 250, and preferably ofabout 50 to 150 microns. These materials are also porous and have asurface area of ≧3, and preferably of ≧50, square meters per gram.Catalyst activity or productivity is apparently also improved withsilica having pore sizes of ≧80 Angstrom units and preferably of ≧100Angstrom units. The carrier material should be dry, that is, free ofabsorbed water. Drying of the carrier material is carried out by heatingit at a temperature of ≧600° C. Alternatively, the carrier materialdried at a temperature of ≧200° C. may be treated with about 1 to 8weight percent of one or more of the aluminum alkyl compounds describedabove. This modification of the support by the aluminum alkyl compoundsprovides the catalyst compositions with increased activity and alsoimproves polymer particle morphology of the resulting ethylene polymers.

The copolymer polymerization reaction is conducted by contacting astream of the monomers, in a gas phase process, such as in a fluid bedprocess as described in U.S. Pat. No. 4,243,619 and substantially in theabsence of catalyst poisons such as moisture, oxygen, carbon monoxide,carbon dioxide and acetylene with a catalytically effective amount ofthe completely activated precursor composition (the catalyst)impregnated on a support at a temperature and a pressure sufficient toinitiate the polymerization reaction.

In order to achieve the desired density ranges in the copolymers it isnecessary to copolymerize enough of the (i) C_(a) comonomers withethylene and the C_(b) comonomers to achieve a level of about ≧0.6 to 9mol percent of the C_(a) comonomer in the copolymer, and (ii) C_(b)comonomers with ethylene and the C_(a) comonomers to achieve a level ofabout ≧0.3 to 7 mol percent of the C_(b) comonomer in the copolymer. Theamounts of the C_(a) and C_(b) comonomers needed to achieve this resultwill depend on the particular comonomers employed.

There is provided below a listing of the amounts, in mols, of variousC_(a) and C_(b) comonomers that are copolymerized with ethylene in orderto provide polymers having the desired density range at any given meltindex. The listing also indicates the relative molar concentration, ofsuch comonomers to ethylene, which are to be present in the recycled gasstream of monomers (C₂, C_(a) and C_(b)) under reaction conditions inthe reactor.

    ______________________________________                                                     Cx/C.sub.2 mol                                                                Ratio in gas phase                                                                         Cx/C.sub.2 mol                                      Cx comonomer at equilibrium                                                                             Ratio in polymer                                    ______________________________________                                        propylene    0.2 to 0.9    0.01 to 0.09                                       butene-1     0.1 to 0.5   0.006 to 0.08                                       pentene-1    0.05 to 0.2  0.005 to 0.07                                       hexene-1     0.02 to 0.15 0.004 to 0.06                                       4-methyl-    0.02 to 0.15 0.004 to 0.06                                       pentene-1                                                                     heptene-1    0.017 to 0.10                                                                              0.003 to 0.04                                       octene-1     0.015 to 0.08                                                                              0.003 to 0.04                                       ______________________________________                                    

The fluid bed reactor is operated at pressures of from about 50 to about1000 psi, and is preferably operated at a pressure of from about 150 to350 psi, with operation at the higher pressures in such ranges favoringheat transfer since an increase in pressure increases the unit volumeheat capacity of the gas.

As earlier disclosed, the at least one layer of the multilayer film ofthis invention comprises heterogeneous copolymers formed from ethylene,and at least one of propylene and butene-1 and at least one C₅ -C₈comonomer. These copolymers generally have a melt index of between about0.5 and about 5.0 decigrams per minute and a density of between about0.91 and about 0.94 grams per cubic centimeter. However, it is preferredthat the copolymers have a melt index of between about 0.5 and about 2.0decigrams per minute and a density of between about 0.926 and about0.930 grams per cubic centimeter because a better balance of physicalproperties is provided to the multilayer film of this invention.Likewise, it is preferred that the copolymer layer of the multilayerfilm of this invention comprise a member selected from the groupconsisting of ethylene-butene-hexene and ethylene-propylene-hexene. Inaddition, the copolymer layer of the multilayer film of this inventionmay be as thick as commercially practical while the other layer orlayers may contain sufficient high pressure, low density polyethylene toprevent melt fracture during bubble cooling at high output rates andalso insure even distribution of the outer layers around thecircumference of the die and bubble. Thus, the copolymer layer may havea thickness of up to about 90 percent of the total thickness of themultilayer film of this invention, and the other layer or layers maycomprise a thickness of about 5 to about 10 percent of the totalthickness of the multilayer film providing that the die design allowseven distribution of the other layer melt or melts around thecircumference of the copolymer layer. However, it is preferred that thecopolymer layer has a thickness of between about 50 percent and about 80percent of the total thickness of the multilayer film of this invention,and the other layer or layers have a thickness of between about 20percent and about 50 percent of the total thickness of the multilayerfilm.

In addition, the multilayer film composition of this invention maycomprise a first film layer of heterogeneous copolymers formed fromethylene and at least one of propylene and butene-1, and at least one C₅-C₈ comonomer; and a second film layer comprising high pressure, lowdensity polyethylene or a blend of said high pressure, low densitypolyethylene and said aforementioned heterogeneous copolymers. When soconstructed, it is preferred that said first film layer comprise up toabout 70 percent of the total thickness of the multilayer film and saidsecond film layer may comprise the remaining thickness of the totalthickness of the multilayer film. In similar fashion, the multilayerfilm composition of this invention may comprise a first film layer ofheterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer, to which acolorant selected from a pigment or dye such as green or black may beadded; a second film layer comprising heterogeneous copolymers formedfrom ethylene and at least one of propylene and butene-1 and at leastone C.sub. 5 -C₈ comonomer; and a third film layer comprising highpressure, low density polyethylene or a blend of said polyethylene andsaid heterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer to which acolorant may also be added. When so constructed, it is preferred thatsaid first and second film layers comprise up to about 70 percent of thetotal thickness of the multilayer film, and said third film layercomprise the remaining thickness of the total thickness of themultilayer film.

Further, the multilayer film composition of this invention may comprisea first film layer of high pressure, low density polyethylene containinga colorant selected from a pigment or dye such as green in color, and asecond film layer comprising heterogeneous copolymers formed fromethylene and at least one of propylene and butene-1 and at least one C₅-C₈ comonomer containing a colorant selected from a pigment or dye suchas black in color and a minor amount of high pressure, low densitypolyethylene. With these materials the thickness of the multilayer filmshould comprise between about 20 to 25 percent from said first filmlayer, and between about 75 to 80 percent from said second film layer.Still further, the multilayer film composition of this invention maycomprise a first outer layer comprising high pressure, low densitypolyethylene or heterogeneous copolymers formed from ethylene and atleast one of propylene and butene-1 and at least one C₅ -C₈ comonomer acore layer comprising high pressure, low density polyethylene orheterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer or a blendthereof, and a second outer layer comprising high pressure, low densitypolyethylene or a blend of high pressure, low density polyethylene andheterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one of propylene and butene-1 and atleast one C₅ -C₈ comonomer, however, it being understood that highpressure, low density polyethylene would not be present exclusively inall layers. Yet further, the multilayer film composition of thisinvention may comprise a first outer layer of high pressure, low densitypolyethylene, a core layer and a second outer layer each of which maycomprise heterogeneous copolymers formed from ethylene and at least oneof propylene and butene-1 and at least one C₅ -C₈ comonomer. In thisconstruction, the heterogeneous copolymers of the core layer and thesecond outer layer may have different densities, different melt indexes,and may contain a colorant such as a pigment or dye as previouslydisclosed. Further still, the multilayer film composition of thisinvention may comprise a first outer layer of high pressure, low densitypolyethylene, a core layer comprising one or more layers wherein atleast one of the layers comprises heterogeneous copolymers formed fromethylene and at least one of propylene and butene-1 and at least one C₅-C₈ comonomer, and a second outer layer comprising heterogeneouscopolymers formed from ethylene and at least one of propylene andbutene-1 and at least one C₅ -C₈ comonomer.

As earlier indicated, the multilayered, coextruded film composition ofthis invention results in a superior, stronger film and trash/garbagebags. In addition, melt fracture during extrusion is reduced. To providefurther reduction in melt fracture during extrusion of theaforementioned 2-layer film structures, it is preferred that thepolymers be extruded, as disclosed by S. J. Kurtz in U.S. Pat. No.4,348,349 and assigned to the present assignee, through a die having adischarge outlet defining an exit die gap formed by opposing die lipsurfaces and wherein one surface of the die lip and/or die land incontact with the molten polymer extends beyond the opposing surface ofthe die lip and/or die land in the direction of the axis of flow of themolten polymer through the die exit whereby melt fracture is reduced onthe surface of the polymer leaving the extended die lip surface. Itshould be noted that use of such extrusion die is not necessary toreduce melt fracture when extruding a 3 or more layered film whereinhigh pressure, low density polyethylene forms the outer layers of thefilm composition. Further, when preparing a 2-layer film structure foruse as a trash or garbage bag in accordance with this invention, it ispreferred that the outside layer comprise the heterogeneous copolymersformed from ethylene and at least one of propylene and butene-1 and atleast one C₅ -C₈ comonomer.

The total thickness of the multilayer film is generally between about 1mil and about 3 mils. Substantially thinner films would usually not besuitable because the strength properties of the film would beunacceptably low for use as a trash or garbage bag. Films substantiallythicker than 3 mils are not preferred since the additional strengthassociated with thicker material is ordinarily not required fortrash-garbage bag usage. A further disadvantage of thicker films wouldbe difficulty in handling and tying the bag open end. A preferredbalance of these opposing considerations is a film between about 1.3 and1.8 mils thick. The other layer or layers of the multilayer film of thisinvention may comprise a high pressure, low density polyethylene havinga melt index of between about 0.5 and about 5.0 decigrams per minute anda density of between about 0.916 and 0.935 grams per cubic centimeter.However, it is preferred that the high pressure, low densitypolyethylene have a melt index of between about 1.0 and about 3.0decigrams per minute and a density of between about 0.918 and about0.922 grams per cubic centimeter because better physical properties areobtained for the multilayer film. High pressure, low densitypolyethylene (HPLDPE) has been commercially available for many years andthe highly branched homopolymer is generally made with a free radicalcatalyst at pressures typically above 15,000 psi, usually in elongatedtubular reactors.

Illustrative, non-limiting examples of the features and practice of theinvention are set out below. The parts and percentages set forth hereinrefer to parts by weight and percentages by weight, respectively, unlessspecifically stated otherwise.

In the following examples, the sample multilayer films were formed byco-extrusion of the film layers through a tubular die.

All the multilayer film compositions were prepared by extruding theouter layers through each of two separate 11/2 inch extruders while thecore layer was extruded through a 31/2 inch extruder. The multilayer dieused to form the extruded bubble was a 12 inch three layer spiralmanifold die. The extruded film flatwidth was 36 inches. The layerthickness ratio of first outer layer to core layer to second outer layerwas 1:3:1 in all the samples prepared. The material employed for thefirst and second outer layers in all the examples was high pressure, lowdensity polyethylene having a melt index of about 2.0 decigrams perminute and a density of about 0.918 grams/cm³. Also the first outerlayer was pigmented with about 5% of a light green pigment, and thesecond outer layer was pigmented with about 4% black pigment. Inaddition, both outer layers contained a total of about 1.2% of slip andanti-block agents. The core layer in all the compositions wasunpigmented and contained no slip or anti-block agent. In Table 1, onlythe compositions of the core layer employed are shown.

                                      TABLE 1                                     __________________________________________________________________________               ETHYLENE-BUTENE-HEXENE      ETHYLENE-BUTENE-1                                                                             CONTROL                           HETEROGENEOUS COPOLYMER     COPOLYMER       HP-LDPE                EXAMPLE    1     2    3     4    5     6    7     8    9                      __________________________________________________________________________    Melt index (dgm/min)                                                                     1.75  2.0  2.0   2.0  1.75  2.0  2.0   2.0  2.0                    Density (gm/cm.sup.3)                                                                    .917  .922 .9235 .9276                                                                              .928  .917 .920  .926 .918                   Puncture Toughness                                                            Load (lb/mil)                                                                            5.4   4.6  4.4   4.5  5.3   3.9  4.5   3.9  3.9                    Energy (in-lb/mil)                                                                       10.5  7.3  7.0   7.3  9.0   6.4  7.6   5.6  3.5                    Elmendorf Tear                                                                Strength (gms/mil)                                                            MD         101   123  99    95   101   82   71    56   359                    TD         503   406  390   403  426   308  316   308  94                     Tensile Strength                                                              (psi)                                                                         MD         2810  3480 2730  3280 3160  2690 2880  2660 2790                   TD         2850  3040 2760  3040 3140  3280 2940  2720 1900                   Yield Strength                                                                (psi)                                                                         MD         1500  1500 1470  1620 1650  1280 1430  1620 1310                   TD         1540  1530 1550  1640 1820  1400 1550  1670 1400                   __________________________________________________________________________

Further, in Table 1, the following test criteria were used. Tensilestrength was measured by ASTM D882 method A. Load at break was reportedin pounds per square inch.

Elmendorf tear strength was measured by ASTM D1992 and is reported ingrams (gms) per mil.

Puncture toughness is a test originated to evaluate the relativeresistance of a film to being punctured. There is no ASTM standard.Basically, a 3/4 inch round steel plunger is pushed through a clampedfilm specimen at a speed of 20 inches per minute using a modifiedInstron Tester. The load to puncture of the film specimen is recorded inpounds (lbs) per mil and the energy to puncture is the integrated areaunder the load-elongation curve and is recorded in inch-pounds(inch-lbs) per mil.

Melt index was determined by ASTM D-1238--Condition E--measured at 190°C. and reported as grams per 10 minutes.

MD signifies machine direction, and TD signifies transverse direction.

Examination of the data in Table 1 shows that a multilayer filmcomposition containing an ethylene-butene-hexene polymer as the corelayer, i.e., examples 1 to 5, has superior and improved physicalproperties over such a multilayer film composition containing as thecore layer either an ethylene-butene copolymer, i.e., examples 6 to 8,or a high pressure low density polyethylene, i.e., example 9.

More specifically, the multilayer films containingethylene-butene-hexene polymer as the core layer (examples 1 through 5)demonstrate that the MD Elmendorf tear strength is essentially unchangedand remains at a relatively high level of 95-123 gms/mil as the densityof the ethylene-butene-hexene polymer increases from 0.917 to 0.928,respectively. This is unexpected in that, generally, as densityincreases MD Elmendorf tear strength usually decreases as demonstratedby examples 6, 7 and 8 which contain core layers of ethylene-butenecopolymers of varying densities. In these examples, the MD Elmendorftear strength decreases from 82 gms/mil to 56 gms/mil as would beexpected as the density of the ethylene-butene copolymer increases from0.917 to 0.926.

The improvements of the core layer products of examples 1 through 5 overthe all high pressure, low density polyethylene control composition(example 9) and conventional copolymers (examples 6-8) are evident whensamples 1 through 9 are compared with respect to important strengthproperties such as Puncture toughness energy, TD Elmendorf tearstrength, and TD Tensile strength.

The advantage of increasing density of the polymers of the core layerwithout lowering MD Elmendorf tear is further highlighted by noting theincrease in TD (trash bag lift direction) yield strength as the densityincreases.

In Table 2, the degree of improvement in various properties provided bythe multilayer films of examples 1 to 5 over control example 9 issummarized. It is to be noted in particular that the greaterimprovements in tensile and yield strength can be obtained withoutsacrifice in machine direction tear strength.

                  TABLE 2                                                         ______________________________________                                        Comparative Results With Control                                              Property       Degree of Improvement                                          ______________________________________                                        puncture toughness                                                            load           +13% to +38%                                                   energy         +100% to +204%                                                 tensile strength                                                              machine direction                                                                            - 2% to +25%                                                   transverse direction                                                                         +45% to +65%                                                   yield strength                                                                machine direction                                                                            +13% to +25%                                                   transverse direction                                                                         +10% to +30%                                                   ______________________________________                                    

Although preferred embodiments of this invention have been described indetail, it is contemplated that modifications thereof may be made andthat some features may be employed without others, all within the spiritand scope of the invention. For example, although the films described inthe foregoing examples were prepared by bubble extrusion, otherpreparative methods may be used as, for example, slot cast extrusion.

What is claimed is:
 1. A coextruded multilayer film comprising a firstouter layer of low pressure, low density gas phase heterogeneouscopolymers formed from ethylene and at least one of propylene andbutene-1 and at least one C₅ -C₈ comonomer, and a second outer layercomprising high pressure, low density polyethylene, said copolymershaving a melt index of between about 0.5 and about 5.0 decigrams perminute, and a density of between about 0.91 and about 0.94 grams percubic centimeter.
 2. A coextruded multilayer film as in claim 1 whereinsaid second outer layer comprises a blend of said polyethylene and saidheterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer.
 3. Acoextruded multilayer film as in claim 1 wherein said first outer layercomprises more than one layer of said heterogeneous copolymers.
 4. Acoextruded multilayer film as in claim 3 including a colorant in atleast one of said layers.
 5. A coextruded multilayer film as in claim 1wherein said copolymers have a melt index of between about 0.5 and about2.0 decigrams per minute and a density of between about 0.926 and about0.930 grams per cubic centimeter.
 6. A coextruded multilayer film as inclaim 1 wherein said first outer layer has a thickness of between about50 percent and about 80 percent, and said second outer layer has athickness of between about 20 percent and about 50 percent of the totalthickness of said multilayer film.
 7. A coextruded multilayer film as inclaim 1 wherein the thickness of said multilayer film is between about 1mil and about 3 mils.
 8. A coextruded multilayer film as in claim 1wherein said high pressure, low density polyethylene has a melt index ofbetween about 0.5 and about 5.0 decigrams per minute and a density ofbetween about 0.916 and 0.935 grams per cubic centimeter.
 9. Acoextruded multilayer film as in claim 1 wherein said heterogeneouscopolymers are selected from the group consisting ofethylene-butene-hexene and ethylene-propylene-hexene.
 10. A coextrudedmultilayer film as in claim 1 wherein said film is formed into a bag.11. A coextruded multilayer film comprising a first layer of lowpressure, low density gas phase heterogeneous copolymers formed fromethylene and at least one of propylene and butene-1 and at least one C₅-C₈ comonomer, a second layer comprising low pressure, low density gasphase heterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer, and a thirdlayer comprising high pressure, low density polyethylene, saidcopolymers having a melt index of between about 0.5 and about 5.0decigrams per minute, and a density of between about 0.91 and about 0.94grams per cubic centimeter.
 12. A coextruded multilayer film as in claim11 wherein said third layer comprises a blend of said polyethylene andsaid heterogeneous copolymers formed from ethylene and at least one ofpropylene and butene-1 and at least one C₅ -C₈ comonomer.
 13. Acoextruded multilayer film as in claim 11 including a colorant in atleast one of said layers.
 14. A coextruded multilayer film as in claim11 wherein said film is formed into a bag.
 15. A coextruded multilayerfilm comprising a first outer layer of high pressure, low densitypolyethylene, a core layer comprising one or more film layers wherein atleast one of said film layers consists essentially of low pressure, lowdensity gas phase heterogeneous copolymers formed from ethylene and atleast one of propylene and butene-1 and at least one C₅ -C₈ comonomer,and a second outer layer consisting essentially of low pressure, lowdensity gas phase heterogeneous copolymers formed from ethylene and atleast one of propylene and butene-1 and at least one C₅ -C₈ comonomer,said copolymers having a melt index of between about 0.5 and about 5.0decigrams per minute, and a density of between about 0.91 and about 0.94grams per cubic centimeter.
 16. A coextruded multilayer film as in claim15 wherein said film is formed into a bag.
 17. A coextruded multilayerfilm as in claim 15 wherein said first outer layer comprises a blend ofsaid high pressure, low density polyethylene and said heterogeneouscopolymers formed from ethylene and at least one of propylene andbutene-1 and at least one C₅ -C₈ comonomer.
 18. A coextruded multilayerfilm as in claim 15 including a colorant in at least one of said layers.19. A coextruded multilayer film as in claim 15 wherein the thickness ofsaid multilayer film is between about 1 mil and about 3 mils.
 20. Acoextruded multilayer film as in claim 15 wherein said high pressure,low density polyethylene has a melt index of between about 0.5 and about5.0 decigrams per minute and a density of between about 0.916 and 0.935grams per cubic centimeter.
 21. A coextruded multilayer film as in claim15 wherein said heterogeneous ethylene copolymers are selected from thegroup consisting of ethylene-butene-hexene andethylene-propylene-hexene.